Make no mistake, this ban will not affect the Nigerian leadership class whose largely responsible for this ban. See reasons for the ban (Full Report: US Visa Ban on Nigeria. http://www.cvakutamedia.com/news-around-the-world/full-report-us-visa-ban-on-nigeria/).
Most Nigerian leaders who are interested in US citizenship would have acquired it by now or done so for their children. Most of them have valid US visa etc. So many travel with diplomatic passports. Banning issuance of immigrant visa will only affect struggling innocent Nigerians, who would have been mainly from the Southern part of Nigeria, because that is the region that has more of their indigenes residing at America and other Western countries.